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  <div class="question_difficulty">
   难度：Easy
  </div>
  <div>
   <h1 class="question_title">
    110. Balanced Binary Tree
   </h1>
   <p>
    Given a binary tree, determine if it is height-balanced.
   </p>
   <p>
    For this problem, a height-balanced binary tree is defined as:
   </p>
   <blockquote>
    <p>
     a binary tree in which the depth of the two subtrees of
     <em>
      every
     </em>
     node never differ by more than 1.
    </p>
   </blockquote>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <p>
    Given the following tree
    <code>
     [3,9,20,null,null,15,7]
    </code>
    :
   </p>
   <pre>
    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    Return true.
    <br>
    <br>
    <strong>
     Example 2:
    </strong>
   </p>
   <p>
    Given the following tree
    <code>
     [1,2,2,3,3,null,null,4,4]
    </code>
    :
   </p>
   <pre>
       1
      / \
     2   2
    / \
   3   3
  / \
 4   4
</pre>
   <p>
    Return false.
   </p>
  </div>
  <div>
   <h1 class="question_title">
    110. 平衡二叉树
   </h1>
   <p>
    给定一个二叉树，判断它是否是高度平衡的二叉树。
   </p>
   <p>
    本题中，一棵高度平衡二叉树定义为：
   </p>
   <blockquote>
    <p>
     一个二叉树
     <em>
      每个节点&nbsp;
     </em>
     的左右两个子树的高度差的绝对值不超过1。
    </p>
   </blockquote>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <p>
    给定二叉树
    <code>
     [3,9,20,null,null,15,7]
    </code>
   </p>
   <pre>    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    返回
    <code>
     true
    </code>
    。
    <br>
    <br>
    <strong>
     示例 2:
    </strong>
   </p>
   <p>
    给定二叉树
    <code>
     [1,2,2,3,3,null,null,4,4]
    </code>
   </p>
   <pre>       1
      / \
     2   2
    / \
   3   3
  / \
 4   4
</pre>
   <p>
    返回&nbsp;
    <code>
     false
    </code>
    。
   </p>
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